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Cybersecurity

Firewall, IDS & IPS: Security Guide

Firewall types, iptables/UFW rules, network zones, IDS vs IPS, Fail2Ban, ModSecurity WAF, and log analysis.

#firewall #ids #ips #waf #network-security

Back to Notes Library

Firewall, IDS & IPS: Complete Security Reference

Overview of Network Security Controls

Control	Role	Acts On
Firewall	Filter traffic by rules	Allow / Block
IDS (Intrusion Detection System)	Detect threats, generate alerts	Alert only
IPS (Intrusion Prevention System)	Detect and block threats	Block automatically
WAF (Web Application Firewall)	Filter HTTP/HTTPS traffic	Allow / Block at Layer 7
SIEM	Correlate logs and alerts	Analyze and alert

Firewall Types

Type	OSI Layer	Inspects	Strengths
Packet filter	Layer 3–4	IP, port, protocol	Fast, simple
Stateful inspection	Layer 3–4	Connection state	Tracks sessions
Application (proxy)	Layer 7	Payload content	Deep inspection
NGFW (Next-Gen)	Layer 3–7	App, user, threat intel	Unified policy
WAF	Layer 7	HTTP headers, body, URL	Web app protection

Firewall Rule Concepts

Packet Filter Rules (iptables)

bash

# List current rules
iptables -L -v -n

# Allow established connections (stateful)
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow SSH from specific subnet only
iptables -A INPUT -p tcp -s 192.168.1.0/24 --dport 22 -j ACCEPT

# Allow HTTP and HTTPS
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Drop all other inbound traffic (default deny)
iptables -A INPUT -j DROP

# Allow outbound (permissive for now)
iptables -A OUTPUT -j ACCEPT

# Log dropped packets before dropping
iptables -A INPUT -j LOG --log-prefix "DROP: " --log-level 4
iptables -A INPUT -j DROP

Linux UFW (Uncomplicated Firewall)

bash

ufw default deny incoming
ufw default allow outgoing

ufw allow 22/tcp          # SSH
ufw allow 80/tcp          # HTTP
ufw allow 443/tcp         # HTTPS
ufw allow from 10.0.0.0/8 to any port 3306   # MySQL from internal only

ufw enable
ufw status verbose

Network Zones (Defense in Depth)

text

Internet
    │
    ▼
[Edge Firewall / NGFW]
    │
    ├── DMZ (Demilitarized Zone)
    │   ├── Web Server
    │   ├── Mail Server
    │   └── Load Balancer
    │
[Internal Firewall]
    │
    ├── Application Zone
    │   └── App Servers
    │
    ├── Database Zone
    │   └── DB Servers (no direct internet)
    │
    └── Management Zone
        └── Admin hosts, logging, monitoring

IDS vs IPS

Feature	IDS	IPS
Position	Out-of-band (network tap)	Inline (traffic must pass through)
Response	Alert / log only	Block + alert
False positive risk	Low impact (just an alert)	High impact (blocks legitimate traffic)
Performance impact	Minimal	Higher (latency)
Deployment difficulty	Easier	Requires careful tuning

Detection Methods

Method	How It Works	Strengths	Weaknesses
Signature-based	Match known attack patterns	High accuracy for known threats	Misses zero-days
Anomaly-based	Detect deviation from baseline	Catches novel attacks	High false positive rate
Policy-based	Violates defined security policy	Customizable	Requires manual policy definition
Heuristic	ML/statistical analysis	Adaptive	Complex to tune

Popular IDS/IPS Tools

Tool	Type	Deployment	Open Source?
Snort	Network IDS/IPS	Inline or passive	Yes
Suricata	Network IDS/IPS	Inline or passive	Yes
Zeek (Bro)	Network traffic analyzer	Passive	Yes
OSSEC/Wazuh	Host-based IDS (HIDS)	Agent on hosts	Yes
Fail2Ban	Host-based (log analysis)	Agent on server	Yes

Fail2Ban (Common HIDS Setup)

ini

# /etc/fail2ban/jail.local
[DEFAULT]
bantime = 3600     # ban for 1 hour
findtime = 600     # within 10 minute window
maxretry = 5       # after 5 failures

[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log

[nginx-http-auth]
enabled = true
filter = nginx-http-auth
logpath = /var/log/nginx/error.log
maxretry = 3

WAF (Web Application Firewall)

WAFs operate at Layer 7 and protect web applications specifically.

WAF Type	Examples	Notes
Network WAF	F5 BIG-IP, Barracuda	Hardware/VM appliance
Host-based	ModSecurity (Apache/Nginx)	Open source, flexible
Cloud WAF	AWS WAF, Cloudflare WAF, Fastly	Easy deployment

ModSecurity Core Rules (OWASP CRS)

nginx

# nginx with ModSecurity
load_module modules/ngx_http_modsecurity_module.so;

server {
    modsecurity on;
    modsecurity_rules_file /etc/nginx/modsec/modsecurity.conf;
    # OWASP CRS blocks SQLi, XSS, path traversal, etc.
}

WAF Rule Types

text

Allowlisting: Only permit known-good patterns
  + Lower false negatives
  - High maintenance

Blocklisting: Block known-bad patterns
  + Lower false positives
  - Misses novel attacks

Rate limiting: Block excessive requests
  - Prevents DDoS, brute force, scraping

Geo-blocking: Block by country/region
  - Reduce attack surface for unused regions

Firewall Log Analysis

bash

# View UFW logs
journalctl -u ufw -f

# Analyze denied connections
grep "UFW BLOCK" /var/log/ufw.log | awk '{print $12}' | sort | uniq -c | sort -rn | head 20

# Top source IPs attempting SSH
grep "Failed password" /var/log/auth.log | awk '{print $11}' | sort | uniq -c | sort -rn | head 10

# Active connections (check for unexpected outbound)
ss -tunap | grep ESTABLISHED
netstat -antp | grep LISTEN

Common Mistakes

Default allow firewall rules — always default-deny inbound; explicitly allow only what's needed
No internal segmentation — flat networks mean one compromised host has access to everything
Disabling IPS due to false positives — tune the rules instead; false positives mean the baseline needs calibration
WAF as the only defense — WAFs are a layer; they don't replace patching, input validation, or secure code
Not monitoring IDS alerts — an IDS that no one reads is useless; connect to a SIEM or alerting system

Download Firewall, IDS & IPS: Security Guide

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🧱

Cybersecurity

Firewall, IDS & IPS: Security Guide

Firewall types, iptables/UFW rules, network zones, IDS vs IPS, Fail2Ban, ModSecurity WAF, and log analysis.

#firewall #ids #ips #waf #network-security

Back to Notes Library

Firewall, IDS & IPS: Complete Security Reference

Overview of Network Security Controls

Control	Role	Acts On
Firewall	Filter traffic by rules	Allow / Block
IDS (Intrusion Detection System)	Detect threats, generate alerts	Alert only
IPS (Intrusion Prevention System)	Detect and block threats	Block automatically
WAF (Web Application Firewall)	Filter HTTP/HTTPS traffic	Allow / Block at Layer 7
SIEM	Correlate logs and alerts	Analyze and alert

Firewall Types

Type	OSI Layer	Inspects	Strengths
Packet filter	Layer 3–4	IP, port, protocol	Fast, simple
Stateful inspection	Layer 3–4	Connection state	Tracks sessions
Application (proxy)	Layer 7	Payload content	Deep inspection
NGFW (Next-Gen)	Layer 3–7	App, user, threat intel	Unified policy
WAF	Layer 7	HTTP headers, body, URL	Web app protection

Firewall Rule Concepts

Packet Filter Rules (iptables)

bash

# List current rules
iptables -L -v -n

# Allow established connections (stateful)
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow SSH from specific subnet only
iptables -A INPUT -p tcp -s 192.168.1.0/24 --dport 22 -j ACCEPT

# Allow HTTP and HTTPS
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Drop all other inbound traffic (default deny)
iptables -A INPUT -j DROP

# Allow outbound (permissive for now)
iptables -A OUTPUT -j ACCEPT

# Log dropped packets before dropping
iptables -A INPUT -j LOG --log-prefix "DROP: " --log-level 4
iptables -A INPUT -j DROP

Linux UFW (Uncomplicated Firewall)

bash

ufw default deny incoming
ufw default allow outgoing

ufw allow 22/tcp          # SSH
ufw allow 80/tcp          # HTTP
ufw allow 443/tcp         # HTTPS
ufw allow from 10.0.0.0/8 to any port 3306   # MySQL from internal only

ufw enable
ufw status verbose

Network Zones (Defense in Depth)

text

Internet
    │
    ▼
[Edge Firewall / NGFW]
    │
    ├── DMZ (Demilitarized Zone)
    │   ├── Web Server
    │   ├── Mail Server
    │   └── Load Balancer
    │
[Internal Firewall]
    │
    ├── Application Zone
    │   └── App Servers
    │
    ├── Database Zone
    │   └── DB Servers (no direct internet)
    │
    └── Management Zone
        └── Admin hosts, logging, monitoring

IDS vs IPS

Feature	IDS	IPS
Position	Out-of-band (network tap)	Inline (traffic must pass through)
Response	Alert / log only	Block + alert
False positive risk	Low impact (just an alert)	High impact (blocks legitimate traffic)
Performance impact	Minimal	Higher (latency)
Deployment difficulty	Easier	Requires careful tuning

Detection Methods

Method	How It Works	Strengths	Weaknesses
Signature-based	Match known attack patterns	High accuracy for known threats	Misses zero-days
Anomaly-based	Detect deviation from baseline	Catches novel attacks	High false positive rate
Policy-based	Violates defined security policy	Customizable	Requires manual policy definition
Heuristic	ML/statistical analysis	Adaptive	Complex to tune

Popular IDS/IPS Tools

Tool	Type	Deployment	Open Source?
Snort	Network IDS/IPS	Inline or passive	Yes
Suricata	Network IDS/IPS	Inline or passive	Yes
Zeek (Bro)	Network traffic analyzer	Passive	Yes
OSSEC/Wazuh	Host-based IDS (HIDS)	Agent on hosts	Yes
Fail2Ban	Host-based (log analysis)	Agent on server	Yes

Fail2Ban (Common HIDS Setup)

ini

# /etc/fail2ban/jail.local
[DEFAULT]
bantime = 3600     # ban for 1 hour
findtime = 600     # within 10 minute window
maxretry = 5       # after 5 failures

[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log

[nginx-http-auth]
enabled = true
filter = nginx-http-auth
logpath = /var/log/nginx/error.log
maxretry = 3

WAF (Web Application Firewall)

WAFs operate at Layer 7 and protect web applications specifically.

WAF Type	Examples	Notes
Network WAF	F5 BIG-IP, Barracuda	Hardware/VM appliance
Host-based	ModSecurity (Apache/Nginx)	Open source, flexible
Cloud WAF	AWS WAF, Cloudflare WAF, Fastly	Easy deployment

ModSecurity Core Rules (OWASP CRS)

nginx

# nginx with ModSecurity
load_module modules/ngx_http_modsecurity_module.so;

server {
    modsecurity on;
    modsecurity_rules_file /etc/nginx/modsec/modsecurity.conf;
    # OWASP CRS blocks SQLi, XSS, path traversal, etc.
}

WAF Rule Types

text

Allowlisting: Only permit known-good patterns
  + Lower false negatives
  - High maintenance

Blocklisting: Block known-bad patterns
  + Lower false positives
  - Misses novel attacks

Rate limiting: Block excessive requests
  - Prevents DDoS, brute force, scraping

Geo-blocking: Block by country/region
  - Reduce attack surface for unused regions

Firewall Log Analysis

bash

# View UFW logs
journalctl -u ufw -f

# Analyze denied connections
grep "UFW BLOCK" /var/log/ufw.log | awk '{print $12}' | sort | uniq -c | sort -rn | head 20

# Top source IPs attempting SSH
grep "Failed password" /var/log/auth.log | awk '{print $11}' | sort | uniq -c | sort -rn | head 10

# Active connections (check for unexpected outbound)
ss -tunap | grep ESTABLISHED
netstat -antp | grep LISTEN

Common Mistakes

Default allow firewall rules — always default-deny inbound; explicitly allow only what's needed
No internal segmentation — flat networks mean one compromised host has access to everything
Disabling IPS due to false positives — tune the rules instead; false positives mean the baseline needs calibration
WAF as the only defense — WAFs are a layer; they don't replace patching, input validation, or secure code
Not monitoring IDS alerts — an IDS that no one reads is useless; connect to a SIEM or alerting system

Download Firewall, IDS & IPS: Security Guide

Get this note + 100s more free on Telegram

Join Free →

📱

Get more notes like this daily on Telegram!

Free study notes, cheat sheets & AI tips

Join Free →

10K+ Members Growing Daily

Get Free AI Notes Daily

Join AiTechWorlds on Telegram and get daily AI tips, prompt engineering templates, coding resources, and exclusive content — 100% free!

📚 Free Study Notes🤖 AI Tips Daily⚡ Prompt Templates💻 Coding Resources

Join Free Channel

No spam. Leave anytime.